In accordance with known interpretation techniques, one or more types of porosity-related measurements is combined with measurements of electrical resistivity, or its inverse, electrical conductivity, to infer the character of the fluid content within the pore spaces of a geological formation. Assuming the porous rock matrix is non-conductive, it has been theorized the electrical properties depend only upon the brine or connate water contained in the pores of the rock and the geometry of the pores. The conductivity of a fluid-saturated rock is due to the ions of the dissolved salt that make up the brine and the magnitude of the electrical conductivity has been presumed to be primarily a function of the brine content of the reservoir fluid. Pioneer work in the field was performed by G. E. Archie as set forth in his paper “The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics”, Trans. AIME, v. 146, 1942, PP. 54-62.
As is known in the art, the value of the resistivity of a rock which is completely saturated with brine of a given concentration at a specific temperature was defined by Archie as follows:F=Ro/Rw=Φ−m where F is the formation resistivity factor; Ro is the resistivity of rock 100 percent saturated with brine expressed in ohm-meters; Rw is resistivity of brine expressed in ohm-meters; Φ is the porosity and m is an empirical constant. Resistivities of oil field brines have been investigated and values published for a small range of relatively low temperatures historically encountered during drilling.
Current modeling techniques utilize this information in algorithms to relate the water conductivity to brine salinity, and to use this information to infer the amount of brine present in the reservoir fluid, and thus the hydrocarbon content of the reservoir.